It sounds like marine velvet to me, I had it twice and the fish seemed to peel their skin.
I lost half my big tank and the entire little tank, years apart but from the same thing. Its usually brought in on a fish.
Some thoughts for you. Are you planning one ever putting corals in the tank? Cause you are not supposed to put copper meds in a display tank, they will leech into the seals making keeping inverts and coral not possible. If you are sticking with fish only then you should be ok with that.
Now, what I did when I had the velvet was let the tank sit idle for like 2 months. The fish that survived the first time are for the most part still alive years later. I did not add anything or dose anything I just let nature take its course.
And on a final note, you dont have to cycle with fish anymore.
You can just dose your tank with ammonia either via pure ammonia or with a dead supermarket shrimp to generate the ammonia you need to cycle.
I would SERIOUSLY look into this. Easier on the fish and easier on you. There are a boatload of cycling articles on this site, just do a quick search.